Line voltage compensating timer



July 26, 1966 I F. F. FORBES Q LINE VOLTAGE COMPENSATING TIMER FiledApril 10, 1963 5 Sheets$heet l INVENTOR FRED EFoRBEs W WM ATraRwEYs'July 26, 1966 F. F. FORBES LINE VOLTAGE COMPENSATING TIMER Filed April10, 1963 5 Sheets-Sheet 2 INVENTOR. FRED F. F'0RBE6 WIM Arron/vs? UnitedStates Patent 3,262,356 LINE VOLTAGE COMPENSATING TIMER Fred F. Forbes,Moundsview, Minn., assignor to The Regents of the University ofMinnesota, Minneapolis,

Minn., a corporation of Minnesota Filed Apr. 10, 1963, Ser. No. 272,143Claims. (CI. 8824) The present invention relates to timing devices andmore particularly to an electronic timing apparatus for controlling theoperation of an appliance such as a photographic enlarger by regulatingthe period of time that the appliance operates as a function of theintegral of the line voltage.

It is common practice in most photographic printing and enlargingprocedures to utilize a light source which is operated from a commercialpower line. For a given exposure time, small fluctuations in linevoltage are found to produce relatively large fluctuations in the lightsource intensity which causes the resulting prints to be nonuniform. Itis therefore one objective of the present invention to provide a timerfor photographic use which will compensate for line voltage fluctuationsby integrating the fluctuating light intensity and will turn ofi aphotographic enlarger after a preset amount of light has been collected.

Numerous automatic timing devices of the type described have beenpreviously proposed for photographic enlarging. These prior timingdevices have not however been entirely satisfactory. One prior device ofthe type described includes a photoelectric cell or other lightsensitive unit for receiving light transmitted through the printingpaper. The cell functions to control the duration of exposure as afunction of the light thus collected. The prior devices of this typehave, however, been found susceptible to error due to a variation inlight-transmitting qualities of localized areas from sample to sample.

Another previously proposed device of the type described employs aphotoelectric cell or the like positioned above the printing paper andincluding a lens system for focusing the light reflected by the printingpaper onto the cathode of a photocell. The system has a relatively highcost due to the fact that an optical system, casing and supporting standis required in addition to the stand supporting the illuminating lamp. Afurther disadvantage of many of the previously proposed devices is thatthe electric circuitry required is relatively complicated inconstruction. It is therefore expensive to manufacture and is morefrequently subject-to malfunction.

-In still other prior devices of the type described, a fnaction of thelight produced by the enlarging lamp must be deflected to the lightsensing means.

In view of these and other defects in the prior art, it is thus oneobject of the present invention to provide an improved line voltagecompensating timer of the class described which is relatively simple inconstruction, reliable in operation and can be manufactured at arelatively low cost.

It is yet another object of the present invention to provide an improvedline voltage compensating timer of the class described which does notrequire the use of a light emitting load to illuminate the light sensingmeans.

Still another object of the present invention is the provision of animproved line voltage compensating timer which requires no incandescentlamp as an integral part thereof and can be used for timing theoperation of various devices whether or not they include illuminatinglamps.

A still further object of the present invention is the provision of animproved line voltage compensating timer which is light in weight,compact in construction and can "ice be used for timing the operation ofan appliance simply by making a single electrical connection between thetimer and the appliance.

Other objects of the invention will become apparent as the descriptionproceeds.

To the accomplishment of the foregoing and related ends, this inventionthen comprises the features hereinafter fully described and particularlypointed out in the claims, the following description setting forth indetail certain illustrative embodiments of the invention, these beingindicative, however, of but a few of the various ways in which theprinciples of the invention may be employed. Y

The invention is illustrated by the accompanying drawings in which thesame numerals refer to corresponding parts and in which:

FIGURE 1 is a side elevational view partly broken away of one form ofapparatus embodying the present invention;

FIGURE 2 is a schematic wiring diagram of an apparatus embodying thepresent invention;

FIGURE 3 is a diagrammatic side elevational view a modified form of myinvention;

FIGURE 4 is a diagrammatic side elevational view of another modificationof my invention;

FIGURE 5 is a chart illustrating uncompensated changes in lightintensity as a function of changes in the line voltage;

FIGURE 6 is a chart illustrating a current characteristic of the lightsensing means employed in the present invention as a function of thevoltage at three different light intensities;

FIGURE 7 is a chart illustrating the exposure time established by thepresent invention as a function of the line voltage when connectedv to aphotographic enlarger; and

(FIGURE 8 is a chart illustrating compensated and uncompensated exposurevalues plotted against line voltage.

The present invention will be best understood by reference to thefollowing detailed description and to the accompanying figures whereinthe same numerals refer to corresponding parts of the several views.

Referring now particularly to FIGURES l and 2 there is shown a linevoltage compensating timer according to the present invention. Theapparatus includes a chassis 10 of any conventional and well known typeon which is mounted a light-tight enclosure 12. Current is supplied tothe apparatus through conductors 14 and 16, which are connected to asuitable wall plug 18. Connected across the conductors 14 and 16 is theprimary winding 19 of a transformer 20.

In line 14 is provided a switch 24 which can be used for turning theapparatus on and off. Connected across the conductors 14 and 16 is acurrent controlling device such as a thyratro'n 26. The thyratron 26includes an anode 28, a cathode 30, a control element or grid '32 and afilament 34 which receives current through conductors 36 and 38, thelatter being connected to the secondary winding 22 of the transformer20. The cathode 30 is connected by means of a conductor 40 through aselector switch 42 to the conductor 16. The holding coil 44 of a relay46 is connected in the line 14 between the plug 18 and the anode 28 bymeans of lines 47 and 48. A condenser 50 is connected between the lines47 and 48 and across the coil 44. A condenser 52 is connected betweenthe anode 28 and the cathode 30 A resistor 54 is connected in line 14between the coil 44 and the plug 18.

The relay 46 includes an armature 60 which is yieldably biased towardthe right in FIGURE 2 by a spring 62 and is held thereby in contact witha terminal 64, the latter being connected to a conductor 66. When theholding coil 44 of the relay 46 is energized the armature 60 will moveto a second position shown by the dotted lines in FIG- URE 2, in contactwith a terminal 68 which is connected to a line 70. A safe light or loadoff indicator 72 of any suitable known construction is connected betweenline 70 and the conductor 16. The load off indicator 72 will only beenergized when the current is supplied to the holding coil 44. Connectedto the line 66 is a terminal 74. Another terminal 76 is connected toline 16. Between the terminals 74 and 76 is connected the load orappliance which is, to be operated such as an enlarger lamp 78.

Connected between the conductor 66 and line 16 is the coil of apotentiometer 80. The potentiometer 80 includes a contact arm 82 whichis wired to a condenser charging circuit 84. The condenser chargingcircuit 84 includes a selector switch 86 wired to connect one of a pairof condensers 88 and 90 across a light responsive means such as aphotocell 92 or other light sensitive device. A conductor 94 isconnected to the plates of the capacitors 88 and 90 opposite from thoseconnected to the potentiometer 80. The conductor 94.is connected to aterminal 96 of the selector switch 42 for purposes hereinafterexplained. A conductor 98 is connected between the condenser chargingcircuit 94 and the grid 32.

A resistor 100 is provided in the conductor 98.

The switch 42 includes a first terminal 96 connected to line 94 asdescribed hereinabove. A second terminal 102 connected to line 16 and athird terminal 104 connected to line 40. A contact element 106 issecured to an arm 107 which is mounted on a pivot pin 108. The contactelement 106 will normally be retained in the solid line position ofFIGURE 2 and in this position will contact both of the terminals 102 and104 so as to complete a circuit between conductors 16 and 40. When it isdesired to begin a cycle of operation, the switch contact element 106 ismoved to the dotted line position 110 thereby disconnecting terminal 104from the terminal 102. A spring 112 is connected between the arm 107 foryieldably hiasing the element 106 toward the solid line position of FIG-URE 2. The arm 107 includes a lobe 114 at its lower end which is adaptedto engage -a resilient member such as a leaf spring 116 for normallypreventing a counterclockwise movement of the contact element 106 beyondthe solid line position of FIGURE 2. When it is desired to connectterminals 96 and 102, contact element 106 is moved against the tensionof the spring 116 to the dotted line position 118 of FIGURE 2. Thecontact element 106 will remain in the dotted line position 118 until itis returned manually to the solid line position of FIGURE 2.

As can be seen by reference to FIGURE 1, I position the photoelectriccell 92 a short distance from the thyratron '26 so that it will receivelight radiation emitted from the filament 34. Thus, during operation,the impedance of the cell 92 will vary as a function of the intensity ofthe light incident thereon from the tube 26. I have found that the cell92 need not he placed immediately adjacent to the tube 26 in order tooperate satisfactorily. The cell 92 can, in fact, be placed on theopposite side of the enclosure 12 from the tube 26 so long as it willreceive light directly or indirectly from the tube 26. I have also foundthat the light sensitive surface of cell 92 can be facing in theopposite direction from the tube 26 under some conditions as, forexample, when the enclosure 12 is coated on the inside with a lightreflective material such as White paint.

There are many advantages of employing the tube 26 as the light sourcefor the cell 92. One advantage is that a separate light source such asan incandescent lamp need not be provided, thereby reducing the cost ofthe apparatus. Another advantage of the system described is that anelectronic vacuum tube such as the tube 26 will considerably outlast anincandescent lamp as the light source. In this way I achieve asubstantial increase in the reliability of the system.

Another advantage of the apparatus of FIGURES 1 and 2 is that it canbeused for regulating operation of any appliance whether or not theappliance includes an incandescent light. The apparatus of FIGURES 1 and2 can be, for example, used for timing the operation of an electric'bread toaster.

Refer now to FIGURES 3 and 4 which illustrate a modified form of myinvention. In FIGURE 3, the lamp 78 is positioned above a photographictransparency such as a negative 130. The light passing through thenegative is focused by means of a vertically adjustable lens 132 ontothe upper surface of a sheet of commercially available photographicpaper 134, resting on a sheet of glass 136 afiixed to the top 'of a box138. The box 138 is provided with a highly reflective inside surface andto this end can be formed from highly polished metal or the like. In thealternative, the box 138 can be formed from wood or plastic and linedwith a reflective material such as a metal foil. On one side of theupper surface of the box is a light shield 140. Mounted beneath thelight shield 140 is a photoelectric cell 142, in all respects similar tothe cell 92. The cell 142 is wired in the circuit of FIGURE 2 in theplace of the cell 92. The impedance of the cell 142 will changeresponsive to the light passing from the enlarger lamp 78 through thephotographic paper 134 rather than in response to the light emitted fromthe tube 26 as in the case of cell 92. The reflection of light onto cell142 from the entire inside surface of box 138 has the advantage ofreducing light variations at cell 142 due to localized differences inthickness of the paper 134.

A generally similar arrangement to that illustrated in FIGURE 3 is shownin FIGURE 4 to demonstrate the manner in which a modified form of thepresent invention can be employed for producing contact prints. Thus, inFIGURE 4, the lamp 78 is mounted above a sheet of opal glass 146 whichis placed over a negative 148. Beneath the negative 148 is a sheet ofphotographic paper 150 which itself rests upon a sheet of glass 152affixed to the open end of a box 154. A photoelectric cell 156 ismounted at one end of the box 154. The box 154 and the cell 156 are inall respects similar to those of FIG- URE 3. A light shield 158 ismounted above the cell 156. Again, as in FIGURE 3, the duration ofexposure of the paper 150 when employed as shown in FIGURE 4, willdepend upon the intensity of the light emitting from the lamp 78 andpassing into the enclosure 154. v

As mentioned hereinabove, for a given exposure time, small fluctuationsin line voltage produce relatively large fluctuations in the intensityof light emitted from the illuminating lamp which in turn causes anon-uniformity in the density of the emulsion of onepicture to another.The effect of line voltage changes is graphically presented in FIGURE 5,which illustrates the uncompensated variation in light intensity as afunction of the line voltage in the case of two enlarger lamps as, forexample, a 60 watt lamp at the left and a 150 watt lamp at the right.

FIGURE 6 illustrates the current characteristics of any of thephotoelectric cells 92, 142 or 156 at three light intensities of 1/2L Land 3/ 2L To explain the operation of the cell, it will be assumed thatthe cell is operating at point A where 1:1 E=E and L=L The resistance, Rof the cell will then be given by the equation R =E /I. Therefore, for agiven voltage E, the resistance of the cell will provide a resistance Rin the capacitor charging circuit 84. Therefore, during operation, thevoltage or potential across one of capacitors 88 or 90 will decrease asthe function of the time capacitor C discharges through cell 92, 142 or156 depending upon which cell is used. If the voltage on the capacitor88 at the time of the starting cycle is E Max (point B of FIGURE 6), andat the end of the cycle when the thyratron Voltage has reached E thevoltage is at point C, the resistance of the cell 92 will have decreasedexponentially from a first resistance value to a second value.

- ergizing the safe light 72.

Before a cycle of operation is initiated, the selector switch 86 isadjusted so as to choose whichever of capacitors 88 or 90 will providethe appropriate time range for which operation is desired. Thepotentiometer 80 is then adjusted to select the exact duration ofexposure. Since the grid voltage is increased by elevating the contact82 of the potentiometer 80 in FIGURE 2, the time required to charge thecondenser 88 will be increased and the cycle time willaccordinglyincrease. If the grid voltage is lowered, on the other hand,the time required to charge the condenser 88 will be reduced.

To start one cycle of operation, the lever 107 is pivoted in a clockwisedirection as seen in FIGURE 2 about the pin 108 so that the contactelement 106 moves to the dotted line position 110. The circuit throughcoil 44 is thus temporarily interrupted causing the element 60 to engagecontact 64. It should be understood that the spring 112 will return thecontact element 106 to the solid line position of FIGURE 2 as soon asmanual pressure thereon is released. When the element 60 of the relay 46has contacted terminal64, current will flow through the terminals 74 and76, through the enlarger lamp 78 and also through the potentiometer 80to line 16. Only a very low current will flow through the coil 44 andthis will be insuflicient to withdraw the contact 60 from terminal 64.In the tube 26, a relatively small intermittent direct current will flowbetween the cathode 30 and grid 32 thereby charging whicheverofcapacitors 88 or 90 is connected across the cell 92. The condenser 88,for example, will of the light collected in the cell 92 until the chargeon the grid 32 has reached a value sufficient to trigger tube 26 atwhich time the impedance of tube 26 will drop to a very low value and inturn cause a surge of current between the cathode 30 and the plate 28and through the coil 44. Current in coil 44 will in turn disconnect thecondenser charging circuit 84 and enlarger lamp 78'from the circuit andagain establish a circuit through the safe light 72.

When it is desired-to focus the enlarger lens 132, the element 106 ofswitch 42 is moved to the dotted line position 118, thereby providing asufiicient potential at the grid 32 to reliably maintain tube 26 in theconductive condition. The tube 26 will remain conducting until element106 of switch 42 is again moved to the dotted line position 110, atwhich time a second cycle of operation can be initiated in the samemanner as described above. While various circuit components can beemployed in the present invention, by way of Example I set forth belowseveral of the most important components of the circuit shown in FIGURE2. It is assumed that a 60 cycle 110 volt alternating current supply isemployed:

Resistor 54 3.3K ohms, 1 watt.

Condenser 50 16 ,ufd. 150 volts.

Condenser 52 .01 afd.

Resistor 100 10K ohms.

Condenser 88 4 ,ufd.

Capacitor 90 20 ,ufd.

Photocell 92 La Fayette MS 895, cadmium sulfide photocell.

Potentiometer 80 100K ohms.

Tube 26 6D4 thyratron tube.

' then become charged at a rate which varies as a function With theapparatus according. to the present invention connected to the enlargerlamp 78, changes in the line voltage will be compensated as best shownin curve E of FIGURE 7. Uncompensated changes in light intensity is afunction of line voltage are shown by way of contrast in curve F in:order to emphasize the results achieved.

It is apparent that many modifications and variations of this inventionas hereinbefore set forth may be made without departing from the spiritand scope thereof. The specific embodiments described are given by wayof example only and the invention is limited only by the terms of theappended claims.

What is claimed is:

1. An apparatus for controlling the time interval that electrical poweris available to a load by monitoring the line voltage by means ofgenerating light directly from the line voltage source and coupling thatlight to a light responsive means whose impedance is a-function of thevariation in line voltage, said apparatus comprising in combination; acurrent controlling device having a control element therein, a voltageimpressed upon said control element being adapted to vary the impedanceof said current controlling device; a light source operatively connectedto said current controlling device; a source of electric currentconnected to said current controlling device; said light sourcegenerating light directly from the line voltage source; a condensercharging circuit including a timing condenser and a light responsivemeans conductively connected to said timing condenser, conductor meansbetween said condenser charging circuit and said control element, saidlight responsive means being positionedto receive light from said lightsource, the impedance of said light responsive means being dependentupon the intensity of the light incident thereto from said light sourceand a function of variation in line voltage, a source of electricalcurrent connected to said condenser charging circuit to facilitate thecharging of said timing condenser, whereby the time of charging saidcondenser varies with the change of impedance of said light responsivemeans as a function of the amount of light received thereby from saidlight source.

2. A line voltage compensating timer comprising in combination, acurrent controlling device including a control element adapted toregulate the impedance of said current controlling device, said currentcontrolling device including an incandescent element therein, a sourceof electric current connected to said current controlling de vice, acondenser charging circuit operatively connected to said controlelement, said charging circuit including a timing condenser and a lightsensitive means positioned in light receiving relationship with saidincandescent element, the impedance of said light sensitive means beingdependent upon the intensity of light impinging thereon from saidincandescent element.

3. A line voltage compensating timer comprising in combination, acurrent controlling device having a light value being thereby regulatedby the light emitted from said filament.

4. A line voltage compensating timer for supplying current to a load.bymonitoring the line voltage by means of generating light directly fromthe line voltage source and coupling that light to a photoelectric cellwhose impedance is a function of the variation in line voltage, saidapparatus comprising in combination, a condenser charging circuitincluding a timing condenser and a photo- 7 electric cell connected inparallel therewith, a current controlling electronic tube having a gridconductively connected to one side of said condenser, said electron tubealso having a cathode, means for connecting a source of current acrossthe cathode and the condenser charging circuit whereby said condenserwill become charged responsive to the current between said cathode -andsaid grid, a light source operatively connected to said currentcontrolling tube and connected to said current source to generate lightdirectly from the line voltage source, said light source beingpositioned to direct light on said cell, switch means for interruptingthe current between the cathode andthe grid so that the tube becomesnonconducting, means wired to said tube for energizing said load whenthe current through said tube is interrupted, the impedance of said cellbeing a function of variation in line voltage and controlled by means ofthe intensity of illumination of said cell by said light source, thecharging of said condenser being controlled responsive to the change inimpedance of said cell and the increase of potential on the grid beingadapted to initiate a current through said tube to said means forenergizing said load.

5. The apparatus according to claim 4 wherein said means for engaging.said load comprises a relay having a holding coil connected in serieswith said tube and said source of current.

6. The apparatus according to claim 4 wherein said electronic tubecomprises a thyrat-ron.

7. The apparatus according to claim 5 wherein a potentiometer isconnected between said condenser char ing circuit in said source ofcurrent for regulating the current between said cathode and said grid.

8. The apparatus according to claim 7 wherein said condenser chargingcircuit includes a first and second condenser and a selector switchmeans for selectively connecting said first and second condensers acrosssaid cell.

9. A timing apparatus for a photographic enlarger lamp comprising incombination, a source of current, a thyratron including a grid and anincandescent element therein, a condenser charging circuit including atiming condenser and a photoelectric cell, said cell being posisaidapparatus comprising in combination, a source of current, a currentcontrolling vacuum discharge device including a grid, said enlarger lamp:being operatively connected to said current controlling device andgenerating light directly from the line voltage source, a condensercharging circuit including a timing condenser and a photoelectric cell,saidcell being positioned to receive light from said enlarger lamp andoperatively connected to said grid for controlling the impedance of saiddevice and relay means conductively connected with said device forturning off said enlarger lamp when said device is conductive, theimpedance of said cell being function of variation in line voltage andcontrolled by means of the intensity of illumination of said cell bysaid enlarger lamp, a photograph supporting box positioned in spacedrelationship from said enlarger lamp, said box having a reflectiveinterior and said cell being mounted within said box.

References Cited by the Examiner UNITED STATES PATENTS 2,293,425 8/ 1942Dam-mond 8824 2,481,667 9/ 1949 Holden.

2,749,799 6/1956 Strem 88'24 2,965,814 IQ/r1960 Gartner 8824 X 3,165,9881/1965 Wick et al. 8824 X 3,178,999 4/ 1965 Clapp 8824 NORTON ANSHER,Primary Examiner.

RICHARD A. WINTERCORN, Assistant Examiner.

1. AN APPARATUS FOR CONTROLLING THE TIME INTERVAL THAT ELECTRICAL POWERIS AVAILABLE TO A LOAD BY MONITORING THE LINE VOLTAGE BY MEANS OFGENERATING LIGHT DIRECTLY FROM THE LINE VOLTAGE SOURCE AND COUPLING THATLIGHT TO A LIGHT RESPONSIVE MEANS WHOSE IMPEDANCE IS A FUNCTION OF THEVARIATION IN LINE VOLTAGE, SAID APPARATUS COMPRISING IN COMBINATION; ACURRENT CONTROLLING DEVICE HAVING A CONTROL ELEMENT THEREIN, A VOLTAGEIMPRESSED UPON SAID CONTROL ELEMENT BEING ADAPTED TO VARY THE IMPEDANCEOF SAID CURRENT CONTROLLING DEVICE; A LIGHT SOURCE OPERATIVELY CONNECTEDTO SAID CURRENT CONTROLLING DEVICE; A SOURCE OF ELECTRIC CURRENTCONNECTED TO SAID CURRENT CONTROLLING DEVICE; SAID LIGHT SOURCEGENERATING LIGHT DIRECTLY FROM THE LINE VOLTAGE SOURCE; A CONDENSERCHARGING CIRCUIT INCLUDING A TIMING CONDENSER AND A LIGHT RESPONSIVEMEANS CONDUCTIVELY CONNECTED TO SAID TIMING CONDENSER, CONDUCTOR MEANSBETWEEN SAID CONDENSER CHARGING CIRCUIT AND SAID CONTROL ELEMENT, SAIDLIGHT RESPONSIVE MEANS BEING POSITIONED TO RECEIVE LIGHT FROM SAID LIGHTSOURCE, THE IMPEDANCE OF SAID LIGHT RESPONSIVE MEANS BEING DEPENDENTUPON THE ITENSITY OF THE LIGHT INCIDENT THERETO FROM SAID LIGHT SOURCEAND A FUNCTION OF VARIATION IN LINE VOLTAGE, A SOURCE OF ELECTRICALCURRENT CONNECTED TO SAID CONDENSER CHANGING CIRCUIT TO FACILITATE THECHARGING OF SAID TIMING CONDENSER, WHEREBY THE TIME OF CHARGING SAIDCONDENSER VARIES WITH THE CHANGE OF IMPEDANCE OF SAID LIGHT RESPONSIVEMEANS AS A FUNCTION OF THE AMOUNT OF LIGHT RECEIVED THEREBY FROM SAIDLIGHT SOURCE.